The present invention relates generally to bearing assemblies and more particularly, to a high temperature rolling element bearing assembly which does not require liquid lubrication.
A rolling element bearing assembly typically includes a rolling element bearing and mounting devices, with the rolling element bearing having roller bearings or ball bearings, rolling element retainers (also known as cages or separators), and inner and outer races, and with the mounting devices positioning the inner race with respect to a shaft and the outer race with respect to a housing. Conventional aircraft engine rolling element bearing assemblies are lubricated by oil having a limiting operating temperature of approximately 400 degrees F. Advanced engines for high Mach flight vehicles are projected to have bearing assembly environmental temperatures as high as 1000 degrees F. One approach that has been studied is the use of dry bearings. A dry bearing is a bearing which has no lubrication or has solid lubrication, such as a dry powder. Suggested bearing materials include ceramics such as silicon nitride which can survive high temperature environments.
For instance, in Lubrication Engineering, Vol. 37, No. 7, July, 1981, on pages 407-415 there is disclosed a rolling element bearing assembly in an article entitled "Operation of an All- Ceramic Mainshaft Roller Bearing in a J-402 Gas-Turbine Engine". The inner race is 0.0031 in. loose on the shaft at room temperature but attains a line fit at the maximum design temperature. The outer race is mounted in a compliant ring.
The inventors are also aware of work done in 1984 by others which proposed a dry rolling element bearing assembly design. The assembly included a ceramic inner race apparently held in contact with the metal engine shaft by a metal collar attachment mounted with an interference fit to the shaft and to the outer surface of the inner race allowing a more compliant inner race supporting system. The proposed design also included an outer race attached to a housing through an intermediate spacer material.
However, the ceramic bearing materials have coefficients of thermal expansion which are generally only one-third the value of the coefficients of thermal expansion of the steel shaft or housing. This mismatch in thermal expansion creates a major mounting problem since the ceramic bearing rings (inner and outer races) expand differentially relative to the shaft or housing, and large thermally induced stresses will lead to rapid failure of the more brittle ceramic components. Typically a bearing inner race is mounted on the shaft with an interference between the shaft outside diameter and the inner race bore. This generates a tensile hoop stress which can induce failure by a mechanism of crack propagation. Ceramic materials are less capable than metals of sustaining a tensile hoop stress without fracturing. The combination of low expansion and low fracture toughness makes it difficult to mount any ceramic race.
Another problem with dry rolling element bearing assemblies has been that they require greater cooling than do liquid lubricated bearing systems because of the dry bearings' higher friction which generates more concentrated heat. Also, removing heat from a dry bearing assembly by air cooling is more difficult than removing heat from a liquid lubricated bearing assembly by utilizing the liquid lubricant for heat removal. In Air Force Aero Propulsion Laboratory (Air Force Systems Command, Wright- Patterson Air Force Base, Ohio) Technical Report AFAPL-TR-74-77, Volume 1, Aug. 1974, entitled "High Temperature, High Speed, Solid Lubricated Bearing Technology Phase I--Heat Transfer" by Paul R. Bissett et al., impingement air cooling techniques were applied to solid lubricated rolling element bearing assemblies. Impingement cooling may be defined as cooling with one or more jets of air which strike a bearing surface to be cooled at an angle (ideally ninety degrees) with respect to such bearing surface.
What is needed is a high-temperature rolling element bearing assembly for the engines of advanced flight vehicles.